A geodetic instrument, such as a theodolite, transit, or the like, generally comprises an alidade base and spindle assembly upon which the instrument body, including the standards and telescope, is mounted for rotation about the vertical spindle axis. For purposes of stability and precision the base assembly of such an instrument has commonly been made of steel with the body being mounted directly to the bearing-mounted rotary foundation members in any convenient manner.
In the higher order instruments from which greater precision is expected it has been the practice to utilize metals of substantially similar coefficients of expansion, such as stainless steel and brass, throughout the alidade structure in order to avoid imbalances and distortions due to temperature changes. Particularly susceptible to significant error resulting from the vagaries of expansions are the rotation centers, such as the vertical spindle assembly and the telescope trunnion bearing support assembly.
More recently, the profusion of assemblies and parts comprising the advanced theodolite instrument have led to a requirement for the increased use of lighter structural materials, such as aluminum, in the interest of both economy and maintaining a readily manageable instrument weight. Since there has remained the necessity of utilizing steel in some parts, such as the rotation centers, to ensure precision and instrument longevity, a definite problem has been realized in the interfacing of the steels and aluminums in the instrument structure in such a manner as to prevent errors due to the widely differeing coefficients of expansion of these types of materials.
In some instances attempts have been made to solve the problem by brute force, as by firmly bolting together the parts of dissimilar structural material; however, not only were the expansion stresses sufficiently great that errors nonetheless appeared, but permanent damage to the affected assemblies often resulted. The present invention, on the other hand, provides a means for interfacing the critical theodolite parts of dissimilar material which guides, rather than resists, movement caused by differences in rates of thermal expansion in order to maintain the concentricity of such parts with respect to the axes of rotation. In this manner the previously uncontrollable errors are eliminated from even the higher order instruments.